My DD is a bit like that with my boobs - or the top of them (the area that is above a v necked top in summer) - is automatic almost if she is sat on my lap for her to put hand there. She has tried it on my mum, and also a couple of other close friends (I suppose is nice she feels that comfortable with them!). For her is associated with thumb sucking - so am working on that because she will stop if asked but it happens before she thinks. DS does it too :-( but he is littler so not so bothered.